Most people do not reach for their phone because they suddenly discovered the perfect thing to say. They reach for it because something hurts, feels uncertain or has just triggered the fear that the other person is moving on.
First, work out what you actually want the message to do.
Do you want information? Reassurance? An apology? Another chance? Or do you mainly want the horrible feeling you have right now to stop?
That last one matters. A reply can calm you quickly, but if your mood keeps becoming dependent on whether they answer, how fast they answer and what tone they use, you have handed the same situation another opportunity to control your evening.
Write it somewhere that cannot send it.
Use your notes app, the private Lifted Spirits journal, or an actual notebook. The point is not to make the writing beautiful. Get the exact message out of your head without putting it into their inbox.
If you prefer paper, a simple Moleskine notebook (affiliate link) on Amazon is enough. You do not need an elaborate breakup workbook just to create a gap between feeling something and acting on it.
Notice what happened immediately before the urge.
Maybe you saw them online. Maybe a mutual friend mentioned them. Maybe Sunday evening arrived and the quiet felt heavier. Maybe nothing happened at all and your brain simply went looking for the person who used to regulate that feeling.
When you can name the trigger, the urge becomes information rather than an instruction.
If silence makes you panic, understand why before you chase it away.
If the strongest part is reassurance-seeking, Attached by Amir Levine and Rachel Heller (affiliate link) on Amazon can be useful because it gives language to the chase-withdraw cycle and why distance can make some people want connection even more intensely.
That does not mean every urge is "just attachment" or that your ex did nothing wrong. It means understanding the mechanism can stop you treating every spike of anxiety as evidence that you need to contact them.
Then ask one boring question: will sending this improve tomorrow?
Not the next five minutes. Tomorrow.
If the likely outcome is another ambiguous conversation, another night watching your phone or another round of trying to interpret what they meant, waiting is probably the stronger move. If there is a practical reason you genuinely need to communicate, you can still do that without turning the message into a test of whether they care.